Another option is the "silent" filter used on some 140 applications with B18 and twin SU HS6 carbs.
The filter element is rectangular and flat (about 1/2" thick) and is housed in a rectangular two-piece "pan" where the bottom piece bolts to the carbs and the top piece is held in place with clips.
Once you source the used housing and clips it is a simple matter of getting on the K&N web site and ordering a permanent filter element (by size) to include the fiter service kit. This arrangement will last "forever" and runs much quieter than the round paper filters.
You would also need the short bolts but not the nuts as your car should have the "permanent" nuts on each carb.
The ipd K&N's are the two-hole models and will fit your carbs ok. You will need the PVC adapter for the front carb to accomodate the hose from the oil filler cap.
Cheapo round paper filters are available online mostly on the classic MG and Datsun sites.