This is a tricky one, I'm not sure quite what you're looking for exists without modification. A hollow strap-on might have some structural issues.
The Love Booster might meet part of your needs, but being made of silicone, it might lack the needed rigidity, it's more of a penis extension with a support harness than a traditional strap-on.
There's also the Miss Boss, which has a small sex machine built into it, but it covers the crotch. And I think the crotch plate is load-bearing. Might be an option if you feel up to re-engineering the whole thing, but it would be an expensive method.
This could require a modified or custom-made harness, if you remove the bottom part of the crotch plate, the base the dildo mounts into is more likely to flop around. You might have some luck adding thigh straps to something like the Beginner Pegging Set, or you might be able to add a pair of straps that go around the penis and still link up to the lower strap, but that may be uncomfortable.
Everything I've mentioned is available here on EF, might be worth having a look around the strap-ons available even if modification is needed.
I hope this helps.
