This is version 1 of the hacked copy of the "Jonah" story format that I used in Capri Toot, which I have unimpressively called "Journal". (I'm unaware of what naming pattern the prepackaged Twine formats use, so I cannot follow it.)

Just move the Journal folder inside this zip file into the "targets" folder of your Twine installation to add this to the Story Format menu.
You can also see this example program (modified from 'Town' by Anna Anthropy)

An explanation of some of the CSS and JS is as follows:

#passages { white-space:nowrap; width:auto; height:auto; overflow-x: scroll; overflow-y: hidden; }

white-space:nowrap keeps the passage divs from wrapping when they reach the edge of the #passages div. overflow-x: scroll permits the #passages div to extend to accommodate the addition of passage divs, using a scrollbar.

.passage { display:inline-block; vertical-align: top; white-space:normal; background-color: #eee; font-size: 1.3em; line-height: 175%; margin: 1em; width: 33%; min-width: 35em; height: 56em; padding: 2em; border-width: 1px; border-style: solid; border-color: #333; box-shadow: 0.2em 0.2em 0.2em; }

display: inline-block ensures that the passage divs are arranged horizontally. vertical-align: top ensures that their tops are aligned if they are different sizes. Do note that Journal by default sets the height of passages to 56em, and the width of passages to 33% of the #passages div. This produces the satisfying look of consistent rectangular pages, but of course limits the text you can put in a passage. Feel free to add CSS to extend the height or the width if you wish.

function scrollDivTo(Q, E) {
var D = Q.scrollLeft;
var G = J(E) - Q.offsetLeft;
var C = Math.abs(D - G);
var B = 0;
var I = (D > G) ? -1 : 1;
var F = window.setInterval(H, 25);
function H() {
B += 0.05;
Q.scrollLeft = D + I * (C * Math.easeInOut(B));
if(B >= 1) {
window.clearInterval(F)
}
}
function J(N) {
var O = A(N);
var P = O + N.offsetWidth + N.offsetRight;
var K = Q.scrollLeft;
var L = Q.clientWidth;
var M = K + L;
if(O < K) {
return O - N.marginLeft
}
else {
if(P > M) {
if(N.offsetWidth < L) {
return(O + N.offsetRight - (L - N.offsetWidth))
}
else {
return O
}
}
else {
return O
}
}
}
function A(K) {
var L = 0;
while(K.offsetParent) {
L += K.offsetLeft;
K = K.offsetParent
}
return L
}
};

This is a horizontal replacement for the scrollWindowTo() function in Jonah. Although it takes two arguments, the additional first argument is simply the #passages div. You could probably rewrite it as a drop-in replacement for scrollWindowTo() by just removing the first argument (and adding " var Q = $('passages'); " to the other var definitions) and changing its name to "scrollWindowTo". However, since this was designed to encapsulate scrolling a div, the change of name was necessary in this case.