You might want to try the Innokin Zenith MTL tanks, especially since you've been used to the Protanks. I highly recommend the Zeniths, and they have a large number of other fans here on ECF, too.
They make a 4mL, 23-24mm diameter one; a D22 Zenith which holds 3mL and is 22mm in diameter; and also a smaller D22 that holds 2mL and is 22mm in diameter. They are by far...my absolute all-time favorite drop-in coil tanks. The amount of vapor from them is great, and so is the flavor using the Z-coils! They're all nearly indestructible, top-fill, don't leak, have multiple airflow adjustment holes, are easy-peasy to change out the coil, and many of us get about 2 weeks off each Z-coil. In that time I run about 140-155mLs through it.
The coils come in a variety of resistances, too. Many of us prefer the 1.6ohm ones. Probably the next most popular 1 is the 0.8ohm. There are others, too, that are lower than that, and a new 1.2ohm IIRC. They come in packs of 5, and can be bought for as little as $6.10/box.
IF you'd like to buy 1 to try...if you'll send me a PM, I'll pass along a link to a US vendor who has the full-size, 4mL, Zenith in stainless steel... for less than $9.00 + S/H. It would come with a 0.5ohm coil installed in the tank, but there will also be 1 of the 1.6ohm coils in the box, too. So you could try both. That's a terrific deal because they're normally $19.99 even when on a rare sale. They were regularly about $22-32 for about the first year they were on the market. If it turns out that you like the tank...that vendor has a lot of them, so you should be able to get more if you wanted to at the same really low price.