I see the OP asked this a while back, but also for anyone else looking, the answer actually depends on your budget, criticality of precision cuts, and an understanding of how the tool operates and wears. For instance, the oscillating multitool is very versatile, especially with the variety of blades/attachments available. However, the oscillation is very fast (10k OPM and higher), so there's the potential for a lot of heat that not only can mar what you're cutting, but quickly dull your blades, and even shorten the life of the tool's motor, if you use the wrong blade for your cut. The standard blade that comes with it is usually one used for stabbing cuts (notching out, etc.), not very wide, so unsuitable for the longer cuts in the OP's question -- you'd generate so much heat and wear with that blade after a bunch of longer cuts, you could dull it before your project's finished. You can get a circular wood-cutting multitool blade, which dissipates the heat, plus is not as maneuverable, which you want on longer cuts since the wider shape within what's already cut helps keep it cutting on a good line, and that plus a straight edge to go by could be an option. But you wouldn't want to batch out production runs with it, due to replacement blade costs.
A jigsaw is workable, but due to the blade shape, you want to watch out you don't flex the blade (easy to do with a long, narrow blade that's only attached to the tool at one end), resulting in a wandering cut both in direction and angle. It can be done, but you'll want a straight edge for a fence, and a lot of patience to develop a feel for it that prevents blade flex.
A circular saw is the best inexpensive tool for the job as described, but if the cost is prohibitive, you can as someone suggested get a hand saw or Japanese pull saw. Keeping in mind a few techniques, such as long shallow cuts along the line to establish a shallow 'steering' cut, alternating with steeper cuts to get through the material faster, as well as others easily found online, and you'll be pleasantly surprised at how quickly and easily a sharp saw will get through your project.
But keep asking, and researching. And if you can make a bit of money off the fruits of your labors, you can use that to fund further tool purchases of the proper tool for each job. Good luck!