everythings great cept bloback! no airbox! substitutes?!?

guys,
The entire airbox is missing for my 73 CR250.
Likely thrown away by folks.
I have a mikuni on there right now, and I've got the thing running reasonably well and it has tons of potential, good compression,
everything else is there and it pulls great. but I just can't tune it without some restriction, and I just can't ride in the dust and mud without and airbox.
in fact...it really doesnt' like running with the bare carb very much at all. especially at higher rpms. it is then that I discovered the phenomenon of 'blow back' thru the carb, on non-reeded 2 strokes.
I was using a small pod, and I believe I was saturating it with fuel, which would eventually lead to bad choking.
I am dying to ride it safely, but would like to know if any of you have made substitutes, or used particular filters in the past, that I may base my final jetting on.
I had read online that without a velolcity stack, or some kind of plenum maybe, that the non reed 2 strokes do this naturally.
Can I get a little help?