Fastening a template without clamps

[johnny means]

I would glue it in place with wood glue. After routing just split the MDF of and sand clean. Pictured is a similar operation I do for clamping cauls.

[John Makar]

I love this site. Double-sided with two pin nails worked just peachy.

[Patrick Walsh]

Love double sided tape. I don’t know what I use but I get it at woodcraft. Sadly as I pretty much loath the place.

[David Buchhauser]

With walnut, I would also consider a pin nail. An 18 or 22 gauge pin or two would hold the template fine, and would not be noticeable, unless you had really good eyes and were looking for it. After finish, I doubt even you could find it.

I like the way you think! Great idea!
David

[David Utterback]

A small bead of hot glue on each side of the template in addition to the double stick tape should keep it from shifting. Remove the hot glue with a little denatured alcohol and peel it off. There will be no trace of it left.
Of course if you don't have a hot glue gun, then this suggestion was useless.
Edwin

Not useless since it can be used for future needs. Thanks!