My plan in the same restrictions would be
- with a pencil and ruler, mark out the area to be cut as precisely as possible. Measure multiple times and be sure.
- Go away and come back later
- Redo the measurements while standing on the OTHER side of the workpiece. Are they still right? If not, re-do.
Now you can have confidence your markings are correct.
Drill four holes in the corners of your waste area. Be careful to go straight down, and avoid blowout on the far side. You could do this with a smaller ~4mm pilot bit all the way from one side, and then follow up with a larger drill bit from both sides in turn.
Ideally you want the holes to not-quite touch the lines.
In the middle of your long sides, chain-drill a series of holes so they are inside your line, but just touch. This is where you will get your saw into the work without doing a plunge cut.
Use a Chisel or file to remove any waste that is stopping you from getting the saw into place and lined up for a straight cut.
- If you're going to use the powered saw, get a spare length of timber and check it for straightness on one edge. Put the unplugged saw in the access hole, and take your time lining it all up so the straight edge is in a good position and clamped down. This can be fiddly, so don't rush it.
Also make sure your power saw has a depth of cut sufficient to go through by ~5mm or so. You don't want a lot of spare blade out underneath, or to only cut a trough.
Remember - a powered tool lets you go wrong faster and easier. Take your time.
- If using a hand saw, do the cut in sessions. Aim for a long smooth arm movement using most of the saw blade, and if you hit the line stop, back up the saw and cut straighter. Don't try to "recover" a cut that is off-course. Rest when tired.
Make the cut. Stay INSIDE the line - you'll clean it up later.
If powered, stop moving when your blade enters the corner hole. Let the blade spin to a stop then lift out.
If hand-sawing, you can lift the blade more vertically and cut right to the corner.
Then repeat in the other direction to the other corner.
Then do the same again for the other long side, two separate cuts to each corner.
At this point you have two long very straight cuts and the waste is held at the ends only.
- Add some support under the middle of your work. Moving a sawhorse inward and sit on the work to hold it still.
- Now a choice - you could use your reciprocating saw to cut the shorter cut, or if you feel confident with the circular saw you could repeat the same method that worked for the long cuts. Either way, remember to stay inside the lines.
When one end is cut completely, the waste will move if not supported.
- Reposition so the waste is supported, then do the remaining short cut.
Great - you should now have an undersized hole.
- Check your markings again, and use a coarse file or rasp to remove the high spots. A chisel and hammer can work here too. You could also use a wood plane for the accessible parts.
- At some point you will want to do a dry-fit and see how the bracket goes. Hopefully it doesn't quite fit, and you can see where the binding points are then address those. Repeat till the bracket fits. For very slight material removal, move to a finer file or use sandpaper on a sanding block.
Use the file or chisel to square up the corners if needed. The bracket may have rounded corners already.
Tools you need that you don't yet have or didn't mention:
- Pencil and ruler/measuring tape
- Drill and drill bits. Could be electric but a hand drill is workable. Or a bit and brace.
- Wood File or Wood Chisel+hammer.
- (Optional) a wood plane
- If power saw, then at least 2x clamps. F clamps or G clamps would be fine. A long piece of timber or metal that has a straight edge and a flat side
- Sandpaper, say 60-80 grit for coarse and some 200 grit for final smoothing. And a block to hold it.
Now if you blow the hole and have gaps, use your saw to cut some thin wedges or shims to fill the gaps. There are also gap fillers that come in a cartridge, which would be a good idea to use anyway. Foaming gap fillers will not help, you want a sealant product instead.
Once the hold is done but before final assembly, the wood surface should get a sealant or finish. I normally sand all sides and edges, and round off any sharp corners/edges. Then I dust it and brush on some water-based clear varnish. One coat is enough for items that don't get touched much, but frequently-handled items would get at least 2 coats. Obey dying times as per the can. You might choose a spray paint can finish, but they aren't as hardwearing. Oil-based varnishes are harder wearing again, but they stink and take a lot of prep and dry time so are best avoided.
- Water-based clear varnish, paint brush, water for cleanup, and rags for spills. Newspaper or cardboard to protect the floor. Time to allow varnish to dry undisturbed.
- Gap filler/sealant, and perhaps a cartridge gun. Sharp knife to open the cartridge. Rags for cleanups. Gloves can help too - this can be messy.
If something feels off, stop and reevaluate. Come back here and ask another question, with photos if possible.
Take your time, think ahead, and don't stress. You can do this.