How to make your own custom hardware for OpenWrt

My requirements
- CPU Cores:
Minimum 1
- CPU MHz:
More the better (above 700 Mhz recommended)
- Flash:
Minimum 32 MB (more is better)
- RAM:
More the better (above 256 MB recommended)
- Dual SIM on-board:
Should work at same time active-active together
- WiFi Availability

What do you mean with "custom hardware"? Do you want to actually build something yourself?
Otherwise check out the PCEngines APU3. I don't know how well dual-SIM is supported on OpenWrt, but the APU2 boards work just fine.

yes i want to build my own router

I'd second the APU2/3 boards.

Meconet have some nice cases (I got mine from there), as do Teklager.

I added separate Wireless APs (Ubiquiti), but you can get radio cards (e.g. from Teklager)