... a cheap fix may be to just carry along two 5gal buckets, with holes drilled thru them ... and tie them off to the transom tiedown eyes. It's worth a try, and if it doesn't do the job, then you're not out any $$. Also, if you think or find that the buckets get in your way ... tie them to each side's mooring cleat (if you have them) or run a rope between the two buckets and drape it across the front seat housing and drag the buckets alongside the middle of the boat. Concrete blocks would work, if you have any negative feelings about possibly losing the plastic buckets into your waterways.
If a 10hp is pushing your 15ft'r too fast to effectively troll, even at idle speed, then the boat is probably not drafting very deep to start with. Anything that will cause drag should slow you down some. Any of the things you've mentioned should work ... but would cost $$. The only cheaper alternative, to the buckets/blocks idea, would be to simply shift from idle to neutral, every so often ... but, that can be tiresome :p
.. luck2ya ... cp