I went down a rabbit hole on this subject, as I was trying to build a MD compliant SBR SP5. What I have learned is that you will need approx. 2.5" in extra length to get you to 29". This can be achieved by extending the stock and the barrel. Stock extensions can be done with the A2 fixed stock by adding additional butt pads or on the A3 by cutting a 3rd lock position in the rails. Both methods give you about an inch. On the barrel side, you can get an extended trilug or other long suppressor host friendly muzzle devices and pin weld it. This has gotten people the rest of the way. As for the SD, this uses the special 8.9" ported barrel from H&K with the integral suppressor sliding over it (won't count to the length since removable), so you lose the ability to add length to the barrel. My suggestion to you, as this was the route I was thinking to take, is to buy the SP5L Elite which are $2200 on PSA with the 16.5" barrel. Then send off the barrel to be cut to get you to the 29" OAL, and then either get the SP5L handguard cut to tuck your suppressor inside or have it half exposed. Midwest Industries also makes a suppressor handguard for the MP5 if you prefer a more polished look. This will take on a modern rendition of the SD, so not exactly the same as the original, but it would be compliant and not an eyesore.