Tern's luggage truss has a higher weight rating than Brompton luggage blocks, and since the weight rating is already pretty low, go for Tern's Luggage Truss rather than adapting to Brompton.

I bought Tern's Luggage Truss and their Go-To bag.

There's also their HQ Bag, which is a bit smaller, (not big enough for a 15" laptop) Hold-Em Basket, and Kanga Rack.

The Kanga Rack can have any bag bungeed to it, which is handy, but the bungeeing and un-bungeeing takes extra time. The Basket is perfect for small grocery shopping trips, as it has a handle just for this purpose, just like the store baskets. No need to re-pack when you get out to your bike.

I have a Brompton luggage block on my ZiZZO Forte and it's not as easy to get hooked & unhooked. Vincita and Tern luggage are good quality, but if you're more worried about price, then a Brompton block and a ZiZZO Shopping Bag is also an option.

Here's my Vektron with the truss and Go-To Bag, and a Vincita Big Nash back on the back: