Looking for a plug-in to show Mail subscription box for article pages! Any Suggestion?

Hi, I am running an outdoor blog named Outdoor Fact. This website provides ultimate buyer’s guide and reviews about cool stuff, top-rated gears and products for camping, hunting, fishing or specialized travel. I’ve been using a quite friendly theme provided by MythemeShop which can save me a lot of time of setting up. I can change or turn on/off layout and features in the theme options easily. However, widgets and features of the theme is quite limited. Since there are more and more users access to my contents from SNS and Google, I would like to develop a mail subscription box where I can let users subscribe us via email (for my Email marketing plan later). I would like add a mail subscription box right under Popular Posts widget which can float or anchor when user scrolling down to read because my contents are quite comprehensive and long like this Best RV mattress short queen landing page (https://outdoorfact.com/best-RV-mattress-short-queen). So is there any plugin works like that, please suggest me! Thank you very much!