make up a sobo/neoprene sandwich.......
http://www.ebay.co.uk/itm/SRM-TECH-SORBOTHANE-ISOLATING-DISCS-PACK-8-GREAT-VALUE-/331627752762?hash=item4d368ded3a
http://www.ebay.co.uk/itm/Neoprene-Rubber-Penny-Washers-Adhesive-Backed-M4-M5-M6-M8-M10-M12-SN0001-/250981193606?pt=LH_DefaultDomain_3&var=&hash=item801394491d
on moderate builds a couple of neoprenes will suffice to simply correct AoE, will stick to stock cylinder head well
(M4 - 20mm is the size you seek btw)
BUT for higher 25rps or higher springs a proper sorbo sandwich is perhaps best to adsorb shock better
you will have to make up the sandwich but have a top level of hard wearing neoprene as sorbo will break down
you will also have to make a hole in the sorboathene disc - maybe a little larger than M4 - say 5mm hole
(hole reduces in size as sorbo squishes under pressure/impact)
if making up sorbo sandwich - clean off all self adhesive on neo pads with alcohol, remove stock neo maybe on cylinder head
fuse the stuff together with proper super glue and it should all affix well to the head, slight cushion and AoE sorted
(with sorbo - ensure you check AoE with spring in there as sorbo will compress from 3mm to 1.5mm under load)
pretty sure it is 70d - heck ring/email them to confirm if you wish
or there is some sorbo 70d sheets on fleabay but the sorbo discs are cut to about the right size already
sorbathene isn't cheap but the above is what I recently started to use myself
though tbh I ain't too bothered about re-doing the guns with just neoprene in there
(a 5mm wall of neo is still better than a stock 2mm wall of neo imho)
though just recently I started to use the sorbo sandwich
(Subway is still way better and tastier but makes a mess in ya gun)