As cookie tins are not all that strong of a resonator "box", I'd recommend mounting it so that the neck pierces the round cookie tin in two places, rather than just connecting the neck to one side of the cookie tin. By having the neck pierce the cake tin, the stress on the strings is carried by the neck and will not crumple or otherwise damage your can (assuming you anchor the looped end of the string to the neck and not the can). You will, however, need a bridge to transfer the vibration of the strings to the top of the can.
In regard to neck (or scale) length verse resonator size and bridge placement. The bridge should be some where around or just after the middle of the cake tin. The exact location is usually dictated by the rule "the bridge position is twice the distance from the nut to fret 12 on a chromatic fretboard (or fret 7 on a diatonic fretboard - not counting the 6½ fret)". The string break angle (the angle the strings bend over the bridge) should also be about 15 degrees. The two design rules are from my readings on banjo design. Hope it helps.
So far all my canjos have been the "can on the stick" variety. With round cookie tins, mounting the stick to the inside of the can probably is more aesthetically pleasing, but more difficult to implement. This later approach would likely require a real bridge (not just a hole in the can) and it gives the instrument a more "banjo" appearance. I have seen the same question come up for cigar box guitars (probably on this site), and the main concerns were the ease of construction and the durability of the instrument, as some felt the cigar box with an internally mounted neck would not have the strength and would need additional internal bracing to produce a rugged instrument. But, I don't recall how they felt about the difference in the sound it produced. With a piezoelectric pickup, this shouldn't be much of an issue. The same is likely true for cookie tin canjo. I'd recommend building two versions of the instruments and test them to see which you prefer and then report back here.
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In regard to neck (or scale) length verse resonator size and bridge placement. The bridge should be some where around or just after the middle of the cake tin. The exact location is usually dictated by the rule "the bridge position is twice the distance from the nut to fret 12 on a chromatic fretboard (or fret 7 on a diatonic fretboard - not counting the 6½ fret)". The string break angle (the angle the strings bend over the bridge) should also be about 15 degrees. The two design rules are from my readings on banjo design. Hope it helps.
-RM