I liked the idea of blank tees posted above, so I ordered a range of mens t-shirts to try. The website is good, and the order was dispatched promptly - it arrived 1 or 2 days after I ordered, I forget, but quickly.

Unfortunately most t-shirts were larger than I expected. I ordered large, which is 56 or 57cm depending on the brand. The smallest was 56.5cm according to my old measuring tape, the largest was 60cm. Other than sizing I found the quality of every t-shirt to be good to very good. If you want to wear them without printing something on it you'd probably want the marle colors, which are slightly mottled in a good way, which Marle and Cotton Force both offer. I really liked the quality, comfort, look and value of the Cloke t-shirts and might try them in a medium, though it wouldn't surprise me if they were a bit small.

Here's my notes on the models I tried in case anyone else is interested in trying them.

Gildan 65000 Softstyle Midweight Tee L Charcoal $10.15
A good medium weight. Cotton is not quite as smooth as some of ther others, but well made and comfortable. Has side seams Measures 56.5cm (but feels larger), 56 claimed.

Cloke T101 Outline Tee L Black Marle $14.00
Very nice cotton, black marle looks great. Sizing is quite large, but ok, but the arms are a bit long - to my elbow which I didn't like. Comfortable, good seams, well made. Has side seams. Measures 58.5, 57cm claimed. Returning as too large.

Cloke T401 Edit Tee L Black Marle $16.00
Heavier weight cotton, very nice feel. Grey marle looks nice. Quite large, around the chest, the arms are a bit long - to my elbow which I didn't like. Measured 58.5cm, 57cm claimed. Has side seams.

Cloke T101 Outline Tee M Black Marle 1 14.00

Same as T101 above but medium. Medium is a good size for me, usually I wear large. Arms are still a bit long, but not too bad. Measures 55cm, claimed 54cm. Keeping this one.

Cotton Force T190 Classic Adults Tee L Grey Marle $9.53
Medium weight, slightly thinner than some. Gray marle is a nice mottled grey color rather than a solid boring color. It's large and baggy, the largest of the ones I ordered, both arm length, width and length. Side seams are good, stitching is good. Noting that the same brand has different sizings for their different models. Large 60cm, 57cm claimed.

Cotton Force T300 Icon Mens Tee L Orange $9.84
Medium weight cotton. Orange color is more on the red side. Front of neck is lower than average. Neck stitching is on the outside, which looks a little cheaper than others. Neither fitted nor baggy, fits as expected for a large. Side seams. A bit longer than average but ok untucked.  It's a nice, comfortable t-shirt, the only thing I don't like is the larger head hole and lower neck. Measured large 58cm, 57cm claimed

UCT180 Urban Collab Set Tee L Smoke $19.95
Medium weight cotton, comfortable. Good neck, slightly higher than average which I like, stitching is internal rather than showing. No side seams. Large size fits as expected. Large 57cm, 56.5 claimed. This is the one I was most likely to keep, it's be good with printing, but it's a solid plain color.

U15603 U Flex Pro Style Fitted Cap S/M Grey Melange $22.95
Too small for my fairly average sized head, and elastic rather than adjustable like most hats are.