The Panzerschrek depends on what you want it to do. Would you prefer it to shoot shower grenades, or solid projectiles like mini Nerf rockets?
If the latter, you can buy relatively inexpensive pre-made rocket launchers that take mini Nerf ammo at http://www.rustyspaintballgear.com
under the Paintball section. They have constant air and CO2 cartridge models, the cheapest being a mere $145 USD I believe. You may be able to cannibalize such a device and build it into an authentic looking and fully functional Panzerschrek, or at least draw inspiration from it.
They also have pre-made, functional, Panzerfausts FYI.
Personally I'd like to see an M2HB that's external gas powered in 8mm. You simply can't let a Ma Deuce shoot ammo that's less than half its real bore diameter!