The seller selling the front block for Tern I bought is also selling an adapter to make a bag compatible, but yes its from Aliexpress and you have to make yourself the plate to reinforce the back of the bag if needed.

I trust the block which is a copy of the one of Brompton, same shape, same material, only difference is the mounting points, but less the adapter: compared to the plastic part of the original Brompton bag frame (most Brompton bags use the same type of removable frame made of aluminum tubes inserted in two plastic parts, this frame exists in several sizes, with or without top handle), it seems much weaker

Its also possible to buy an original Brompton bag frame alone and use it to make a bag compatible (probably much more difficult to find in the US than in Europe) or a copy of such frame on Aliexpress which is like the front block a copy of the one of Brompton (and maybe manufactured by the same factory in China!).