I also have a simple topping lift consisting of a line tied at the mast head, back down to the aft end of the boom, thru a cheek block to a horn cleat on the boom. Ive found that even when I tighten up the mainsheet, pull the traveller all the way to port (or starboard) the boom can still swing a bit when motoring or in the slip. If the boom, with furled main, is cnetered over my deployed bimini, it may chafe it. So I took a short piece of 3/8 in line, with a snap shackle at each end, and connect one end to the boom bale and the other end to the toe rail. I loop the toe rail end thru the shackle with a rolling hitch so I can shorten or lengthen it easily. When so connected, this line takes the strain off the mainsheet and topping lift while keeping the boom immobile. I always leave the boat in the slip with this line connected to ensure the boom stays put. It also works as a poor mans preventer when going downwind to prevent accidental jibes. You may want to give this a try.