All you require is good quality multi-layer foam or you can use three different layers of foam (varying filtration size)
Good Quality Aluminum or Stainless Steel Mesh
A pair of Scissors
High Quality Glue (VERY IMPORTANT)
First Step !1
Get Good quality Foam .........
STEP 2 .
CUT THEM INTO APPROPRIATE SIZES LENGTH ways .......Longitudinally
STEP 3.
CUT THE ALUMINUM WIRE MESH IN THE SAME WAY PAYING ATTENTION TO YOUR TRUMPET LENGTH AND DIAMETER
STEP 4.
ROLL THEM UP ROUGHLY AS BIG AS DIAMETER OF THE ITB (40-45mm) and USE HIGH QUALITY WIRE TO HOLD THEM TOGETHER BY TWISTING THEM.
STEP 6.
ROLL THE LAYER OF FOAM ON TOP OF THE MESH CUTTING OFF THE EXCESS AND GLUING TOGETHER THE SEAM/EDGES ...AND CUT A HOLE IN THE REMAINING FOAM TO FORM THE TOP .....AND GLUE IT IN PLACE ....LEAVE FOR 2 HOURS TO SET. THEN USE HIGH QUALITY METAL CLIPS or zipties to secure them onto the ITBs
Enjoy that open ITB orchestra at 10k RPM without sucking in dirt and debris
(WARNING: TRY THIS AT YOUR OWN RISK , I CANNOT BE HELD RESPONSIBLE IF YOUR ENGINE MANAGES TO SUCK SOMETHING FROM IMPROPER FITTING OR INADEQUATE GLUE ETC)