USA spec LHD?  I don't think the OEM RHD air filter will fit with the LHD dual brake master.  That pic looks like they installed the 1968 510 SINGLE master cylinder, to make room for that OEM filter assembly, which is grossly dangerous as a SINGLE brake line master!  With a SINGLE line master, if one brake hard line, hose or hydraulic cylinder ruptures, you simply lose your brakes.  With the 69-73 DUAL master, you will typically have enough brake pressure to at least stop........

There was a write up here on Ratsun some years ago about making one with a similar oval "shape", but the air filter edges were open.......you could see the air filter.  In other words make a flat plate to mount to the carbs, another flat plate for the outside, and they used an OLD 80's (??) Subaru oval shaped air filter or similar, sandwiched in between the two plates.  Problem is you really need a lip around the outer edges facing in, so the filter stays in place.  Hope that helps!

TJ

I got a cold air intake for a MG from Moss motors for my SSS 411 when I changed my brake mc. The spread on the MG carbs is 7.25 as compared to 8.125 for the 411. I cut it in 1/2 and put a silicone boot from Silicone intakes between the 2 pieces. I have a couple of dual carb housings, but the R and L series carb spread are not the same, plus you can't get air cleaners anymore. The only option is to stretch a round air cleaner to oblong, which is a poor one. The roadster elements won't fit. 

Hi, do you know what oval air filtre reference are fitting in?