SparTalk - mast extension sleeve

Hi , I am in the process of lengthening my Zspars mast . a 600e section . I have bought a new 500mm section and today have made 3x 4mm x sleeving pieces to fit inside ,and 1x for the outside ( fwd .) total of 4 length x 500mm . which have been formed to fit .I have got monel rivets 6mm x 17mm to rivet the lot together . I thought it worth running by Spartalks geniuses for comment . a few details , new section going at the top . fractional rig ,cat ,mast length 13100 mm . main and kite flown from mast head . 6 hours spent making a nice butt joint to the mast after cutting level where the crane fitted in ,( 80mm ). so net added length is actually 420mm . this is all I needed. however I found that the lazy tongs rivet gun ,although pretty hefty , will not " pull " the sleeve tight to the mast . I was considering using self tappers to do this and removing them afterwards. I read here that self tappers should not be used for permanent fixing and would agree .I also wanted to use a bonding agent too as there are some very clever products around for this but I am on a Greek island where it took months to find some aluminium ! ( a roadsign !) so would it be worth using epoxy ? which is available here. as its a short section i can actually put nuts and bolts in it too . I am thinking about an extra sleeve inside fwd so making a sandwich of the mast ? is this overkill ? or insurance ? I would be grateful for opinions on this little project , out on a limb ,here almost no yachties to chat with about this kind of thing . , gramos