An update that may be helpful to the original poster and/or others:
I did manage to purchase a longer receiver - a #4, but it only partially solved my problem, because when I inserted it into my $80 custom molds, I discovered that the molds were so (poorly?) designed that they did not “swallow” up the right ended bend in the receiver wire and so it stuck straight out into the air from the mold instead of being inside the part of the mold that went inside my ear canal. Not only was this very annoying and looked ridiculous, but it perpetuated the feedback screeching problem.
I then tried heating the right angle wire and flattening it against the custom mold but the flattening would only last a couple of days and then return to its right-angle bend, just as others who had tried this reported.
I then bought some “tulip” tips on eBay that were supposed to allow me to implant the right angle bend fully into the ear canal but they kept slipping out just like the standard dome tips, and after slipping partway out the feedback screeching would return every time I turned my head to the right or left!
I then decided out of desperation to try one more thing, and it worked! I found a kit called “Hearing Aid Domes - Universal Domes for Hearing Aids - Sizes Small, Medium, Large & X-Large Earbud Replacements” on Amazon" 39 pieces in different sizes for only $12.99. Despite the claims of hearing aid dispensers that, supposedly every brand of hearing aid requires custom fittings, these include many different sizes and several shapes and claim to be a “universal fit.”
When I got them and tried out various sizes and shapes, I found that the internal tube into which you insert the receiver is very snug and quite deep, so that it accepts not merely the metal end of the receiver but also the right-angle bent part of the attached wire, thus allowing me to comfortably fit the bent part fully into the ear canal (encased inside the soft tube) where it is meant to go, and it stays there, does not slip out, and totally eliminates both the feedback screeching and also the goofy look of having the right angle bend of the wire sticking straight out of my ears into the air!
In addition, my subjective impression of my hearing with these tips is that it is equal to or possibly even a little better than it was with those custom molds.
Works great! https://tinyurl.com/yx9natfr
Note that most of the reviews are quite positive, except for the ones who stated that they couldn’t find any of the tips to fit their particular hearings aids. The only negative one that concerned me at first was one guy that complained the tip came off in his ear, but I cannot imagine how that could happen unless he failed to insert the receiver fully into the long, snug, deep internal opening. Once inserted, no way is that going to slip off by itself!