The high page rank since this question was asked indicates that many either want to know the answer, or like the drill-out hack. (Ahem, "adaptation.")
Electricians have been asking for a weatherproof, "listed" SER cable fitting for at least two decades. At this writing, none are yet available.
There is a family of fittings that mirrors those for SEU (flat/oval-shaped cable, the connector for which was used in the hack), except with round elastomeric seals. They are typically sold under the title "cord grip" connector; are listed for use with a variety of cables, but not SER; and are not widely stocked in SER sizes. The only detectable reason these aren't listed for use with SER is the lower temperature rating of the seal material, 140ᵒF (60ᵒC). The metal parts look identical to those of SEU connectors.
So with both the metal and the seals in these fittings (SEU and cord grips) in active production, why does any manufacturer not just make a temperature-suitable, round seal that fits SER, assemble with existing metal parts, get it listed for SER and sell a million, thereby making everyone's life easier? Three possible reasons.
First, in the longterm absence of this needed widget, alternatives have been developed. Electricians and inspectors have done a delicate dance around this item for many years because no single fitting exists that as-is meets code. The most-used workaround has been to install a strap-type clamp not listed for wet locations and just glob it up with 'duct seal' (the clay-like, hand-applied sealant that's supposed to remain permanently soft, but years after installation is sometimes seen to crumble and fall off). So manufacturers may fear low sales resulting from the continuation of what has become the status quo (the inferior attributes of which should drive the market).
Second, SER is called "round," but it's actually a 'bumpy-round' or lobular-round. Seal materials with higher temperature ratings may be less compliant and unable to completely close between the lobes or bumps. But the deviation from perfect circularity in SER is relatively small so this possible reason carries the least weight.
Third, SER in single-phase use (residential) is 'feeder' cable. Feeders go from the main disconnect panel to a 'sub-panel'. The great mass of residential installations don't (currently) use feeders because the inside panel is the main disconnect. No feeders needed. (For most.)
However, that's changing. With NEC 2020, an outside disconnect is required, which will typically necessitate a feeder cable from the outside box (meter main or 'combo'), through our fitting, to the inside breaker panel. So while demand for these fittings will be increasing as more jurisdictions adopt the 2020 and later codes (NEC is updated every three years), market pressure currently exists only as potential since no product is available for purchase. (Come on, manufacturers, catch the leading edge of the wave!)
So all of the above is nice to know but, as asked, what fitting to use?
- The hack in the accepted answer should be acceptable in practical use. (But a fitting with a drilled out seal could certainly never be listed!) Technically, duct seal could be forced into any tiny voids around the cable at the seal entrance and if a good job had been done with drilling, the fitting should be functionally the same as if the cable were SEU. But in any case, inspectors know that listed, watertight SER fittings don't exist so should spot the "adaptation" instantly, and maybe reject it. For that reason (and I hate to beat the drum here), it's always better to check with the "AHJ" (local inspector) before proceeding.
- The other alternative is, of course, the strap ('squeeze') fitting mentioned above (which again, is listed only for dry location use when unmodified). Some electricians cleverly turn the clamp end of the fitting to the inside of the box (nut to the outside). Duct seal is then packed into the external void around the cable until flush with the end of the fitting;
- Looks better now, much more finished and professional in appearance
- Keeps the electroplated steel parts from corroding over time, which
will look better in the future
- Has the disadvantage of sacrificing
valuable space inside the panel needed for cable positioning
Having experienced the same frustration as @David Pfeffer in trying to find a proper fitting, sharing this info has been cathartic. Hope it saves some time and grief for many.